The present invention relates to a transparent cover plate of a synthetic plastic material for a headlight of a motor vehicle and other light sources, which is provided at least at its outer side with a coating of a transparent synthetic plastic, as well as to a method of producing the transparent cover plate.
Cover plates for motor vehicle headlights are usually composed of glass. For the reason of accurate shaping of dispersing and lens elements as well as for the reason of the weight saving, cover plates are increasingly produced of synthetic plastic materials. Synthetic plastic materials suitable for this purpose are usually polycarbonate, aromatic modifiers polycarbonate (APEC) and polymethylacrylmethylimide (PMMI), which however require a surface improvement to satisfy corresponding requirements with respect to scratch resistance, stone impact resistance, weather resistance and resistance to solvents, fuels and oil.
German document DE-A-27 48 358 discloses a method of improving the surface layer of the cover plate of synthetic plastic material. The treatment is performed directly on the synthetic plastic of the cover plate, for example in form of a subsequent hardening.
In older time the coating of the synthetic plastic cover plates with a hardenable lacquer was used. The lacquer is applied in several layers until the desired layer thickness is formed. Lacquer resins are based of cross-linkable acrylate, polyester, polysiloxane. Systems are known in which the pre-polymerized cross-linkable synthetic plastic is dissolved in organic solvents. Furthermore, aqueous systems on acrylate bases are known. Also, already proposed, solvent-free systems are utilized, in which a pre-polymer is dissolved in aqueous monomers. The monomers used as solvents are then bonded in the lacquer layer during hardening.
The known coating processes are complicated and pose environmental problems and health risks. Partially, also the desired or required mechanical properties, weathering properties and resistance against chemicals and solvents as well as optical properties are not satisfactory.